Satellites help study dining habits of sea lions, seals
Posted: December 9, 2001

Figuring out what a northern fur seal has eaten recently can be a messy business, says fisheries biologist Jeremy Sterling of the National Oceanic and Atmospheric Administration's National Marine Mammal Laboratory in Seattle, Wash. Determining where they have been eating is a bit more high-tech.

The Alaska Ecosystems Program, which includes Sterling and more than a dozen other researchers, has been using satellites to track northern fur seals and Steller sea lions in the Gulf of Alaska and the Bering Sea for nearly a decade. With a transmitter attached to an animal's back sending signals to the Argos instrument on the National Oceanographic Atmospheric Administration's polar-orbiting satellite, the scientists can record the marine mammal's path as it swims hundreds of kilometers, or miles, from land foraging for food.

Now the scientists are combining their information about where the animals go to find food with data from another satellite, Topex/Poseidon, to understand why the animals choose to go where they do. NASA's Jet Propulsion Laboratory, Pasadena, Calif., manages the Topex/Poseidon program for NASA's Earth Science Enterprise, Washington, D.C.

With fewer than 40,000 individuals, Steller sea lions are an endangered species. Northern fur seals now number less than 1,000,000 in U.S. waters. The Alaska Ecosystems Program monitors both species and conducts research on the animals' numbers, health, mortality and basic life histories--where they live, how they reproduce, how well they survive. What the scientists learn is important in deciding how best to protect the animals and manage the fisheries on which they and many other species, including humans, also depend.

"We'd like to see the populations increasing and looking well," says Sterling, "but both are declining. Our job is to try to figure out why. Is there a climatic change? Is there a disease? Are the animals not able to reproduce? Are they not getting enough food? Has there been some change in the food base?"

"We use lots of different methods to find out what's going on," says Sterling. "Members of our research group go out and count animals, note seasonal and annual changes, weigh pups. Satellites are another tool."

While satellite tracking provides information about where the animals go for food, the researchers wanted to know more about the environment where and when the seals and sea lions were foraging. "We want to figure out why animals go to certain areas, what affects their decision-making,"says Sterling.

They turned to Topex/Poseidon altimeter measurements of the Bering Sea provided by Dr. Robert Leben, research associate professor, Colorado Center for Astrodynamics Research, University of Colorado. Topex/Poseidon measures sea surface topography, the hills and valleys of the ocean's surface, revealing the location of currents and eddies. The altimeter data also indicate ocean temperature and other physical conditions.

The researchers combined some of their tracking data from 1999 and 2000 with Topex/Poseidon measurements made during the same periods. "We can definitely see some patterns, " says Sterling. "It appears that the animals travel on the edges where the eddies and gyres occur. Eddies concentrate food where animals can feed."

Topex/Poseidon has been making continuous measurements of sea surface height since 1992. Jason 1, launched on December 7, will carry on Topex/Poseidon's mission of monitoring the globe's oceans even further into this century. Like Topex/Poseidon, Jason 1 is a joint project between NASA and France's Centre National d'Etudes Spatiales.

"We've been tagging animals since the early 1990s," says Sterling. "We now have the potential to go back and see what patterns we can find that can help us understand the animals' behavior. We can also get new maps of the Bering Sea every three days to use with our current studies. "

"Our goal is to learn everything we can to help us manage these species and the fisheries," he says.

JPL is managed for NASA by the California Institute of Technology in Pasadena.